util/syspolicy: finish plumbing policyclient, add feature/syspolicy, move global impl

This is step 4 of making syspolicy a build-time feature.

This adds a policyclient.Get() accessor to return the correct
implementation to use: either the real one, or the no-op one. (A third
type, a static one for testing, also exists, so in general a
policyclient.Client should be plumbed around and not always fetched
via policyclient.Get whenever possible, especially if tests need to use
alternate syspolicy)

Updates #16998
Updates #12614

Change-Id: Iaf19670744a596d5918acfa744f5db4564272978
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

client/web/web_test.go

@@ -28,6 +28,7 @@ import (
 	"tailscale.com/tailcfg"
 	"tailscale.com/types/views"
 	"tailscale.com/util/httpm"
+	"tailscale.com/util/syspolicy/policyclient"
 )
 
 func TestQnapAuthnURL(t *testing.T) {
@@ -576,6 +577,7 @@ func TestServeAuth(t *testing.T) {
 		timeNow:     func() time.Time { return timeNow },
 		newAuthURL:  mockNewAuthURL,
 		waitAuthURL: mockWaitAuthURL,
+		polc:        policyclient.NoPolicyClient{},
 	}
 
 	successCookie := "ts-cookie-success"